L-carnitine is an essential substance for mediating long-chain fatty acid transport through the mitochondrian membrane prior to intracellular .beta.-oxidation, and hence a deficiency of L-carnitine causes disorders in fatty acid and its related metabolisms. Particularly, it is believed that disorders of the skeletal muscle and cardiac muscle, both of which are high energy consumption tissues depending on carnitine and lacking in carnitine generation, occur from such deficiency. Heretofore a disease arising from inborn irregularities of carnitine metabolism has been studied, however in recent time, secondary disorders of carnitine metabolism have become a problem in patients suffering from nephrosis and undergoing dialysis. Carnitine is administered to carnitine-deficient patients who have a disease of the body muscle or cardiac muscle, or patients undergoing dialysis. Studies on the behavior of carnitine in diseases and therapy have been required, however a desirable assay method for carnitine in the clinical field has not been developed.